"If you make a sand castle, you won't really see the physical form of oil that can be identified with the naked eye," said Wang, of the University of South Florida (USF) in Tampa.

Markus Huettel, who has been surveying beaches in Pensacola, Florida, monthly, also found that much of the oil has broken down—but he noted that some of the tougher components, such as asphaltenes and resins, will be degraded at a much slower pace.

"It's not that after one year everything is gone," cautioned Huettel, a biological oceanographer at Florida State University (FSU) in Tallahassee. "That's just not possible."